Switchable microwave step attenuators have been previously developed which are intended to provide attenuation at selectively adjustable levels. Typically, the prior art attenuator devices employ a series of attenuator sections designed to provide attenuation in discrete steps. The overall attenuation achieved by such devices is determined by selectively inserting different attenuator sections which, in effect, are connected in series with the microwave transmission line to provide the desired attenuation level.
Although attenuators capable of high attenuation levels have been proposed using the present switch and switching arrangement, in practice, it has been particularly difficult to achieve attenuation as high as 32 dB with minimum frequency sensitivity by the prior art devices. It is desired to provide a switchable microwave step attenuator capable of minimum sensitivity with signals of widely varying frequency, e.g., from DC to 2 gigahertz or more. However, a serious problem arises because of the inherent interelectrode capacitance of the switch in the attenuator which becomes increasingly more troublesome at higher frequencies. As a result, it has been quite difficult to achieve satisfactory linear operations in this type of switchable microwave step attenuators. In instances where linear operation has been achieved, it has been only over limited frequency ranges.
Prior Art references such as U.S. Pat. No. 4,107,633 to Scaletta disclose the use of card-attenuators in transmission and measuring type circuitry. However, when it is attempted to deposit a plurality of separate resistive sections on the same microstrip card and attach switches to each of the resistive sections to switch the resistive sections into and out of the circuit, it is found that substantial parasitic or leakage capacitance is developed across the switched terminals. The leakage capacitance across any given resistive section causes that resistive section to become frequency sensitive. Since in such a programmable attenuator card the attenuation is additive, that is, the values of the attenuator are developed by selecting the proper resistive sections and cascading them, the errors in the individual attenuator sections are also additive. Thus, as the program attenuation is increased, the frequency sensitivity of the attenuation of the card is also increased. In applications where the attenuation accuracy is important, this frequency sensitivity of the attenuation characteristic is completely unacceptable.
An additional reference of interest is U.S. Pat. No. 3,449,697 to Mison which provides an attenuator for use at very high frequencies. The apparatus has a plurality of sliding shields each of which is movable relative to a series resistance of a pi-section attenuator pad of each attenuator section to provide a capacitance which increases or decreases attenuation at high frequencies to compensate for the capacitance effects across switch contacts employed to switch each section in or out. The difficulty with such a device is that the shields can move and, therefore, require constant attention. More importantly, the patent does not provide the information necessary to permit the apparatus to be initially designed and all parameters determined at the time of manufacture and thereafter be free of additional adjustment.
Accordingly, it is an object of the present invention to provide a card-type programmable step attenuator with a monotonic attenuation characteristic even with small--say 0.25 dB--increments of attenuation.
A further object of the present invention is to compensate for the interelectrode capacitance across the switches controlling insertion and removal of attenuator sections in a programmable card-type attenuator.
Another object of the invention is to provide a card-type programmable microwave step attenuator which achieves attenuation in one dB increments from 0 dB to 127 dB and operates with minimum frequency sensitivity over a frequency range from 0 to 2 gigahertz or more.
It is also an object of the invention to provide a switchable microwave step attenuator which incorporates compensation for deleterious capacitive effects encountered at high frequencies without the need for additional circuit components.
A further object of the invention is to provide a card-type, programmable microwave step attenuator which is less complicated in structure than prior art devices and inexpensive to manufacture.
It is a further object of the invention to provide a switchable microwave step attenuator circuit advantageously mounted on a single substrate and wherein the actual circuit components are adapted to provide the required compensation for deviation of the electrical length of each attenuator section from the optimal electrical length.
It is another major object of the present invention to provide a card-type programmable microwave step attenuator having resistive stubs added to the minimum loss circuit when the lower value attenuator sections are withdrawn from operation to render the frequency sensitivity of the device neglectable over a wide frequency range.